PEP in Spontaneous conceptions (PEP-S) represents the sampling of expected normal pregnancies from the local community. Enrolled at the time of the first positive pregnancy test, participants are expected to be included from approximately 5 weeks of gestation. Hereafter participants are recalled every two weeks for blood sampling and transvaginal ultrasonographys of endometrial thickness until a gestational sac and yolk sac are visible for mean sac diameter measuring and eventually the crown-rump-length.